1. Field of the Invention
The invention relates to a method for compensating the sub-divisional error effect by using adaptive tuning for the ebeam inspection tool during continuation scanning.
2. Description of the Prior Art
Defects are inevitably generated in the semiconductor process, which will greatly impact device performance, even failure. Device yield is thus impacted and cost is raised. Current defects can be classified into systematic defects and random defects in general. On the one hand, system defects infer defects will be found repeatedly and systematically in wafers, in which defect patterns can be used as reference in classification to determine root cause of which process incurs such defects. In order to increase semiconductor process yield, it is critical to enhance yield by monitoring, such as by using SEM (Scanning Electron Microscope), systematic defects highly appeared regions in mass production process to real time eliminate systematic defects. On the other hand, the non-systematic defects, random particle defects, are random residues left in wafers. Distributions and characteristic profiles are important references to distinguish systematic defects from non-systematic defects.
In order to enhance semiconductor process yield, defects have to be identified as soon as possible to prevent from impact pouring out. Optical microscope is used in conventional optical inspection which includes bright field inspection and dark field inspection. Every die on a wafer is scanned by optical beam and images of every die are generated and stored. A die-to-die compare is used to identify if there is any abnormal or defect with locations and images thereof.
When semiconductor nodes continue shrinking, dimensions of defect shrink also. Unimportant small defects in previous now become critical therefore. In sub-20 nanometer semiconductor node, optical inspection tool can't reveal any pattern more, even by using interference method, and hence SEM is the only way to identify defects. Nevertheless, due to the detected signal electrons in the SEM are secondary electrons, detection duration inevitably retrogrades significantly compared to that of optical inspection. Hence, it is an important issue to fast identify defects on a wafer by using SEM. An ebeam inspection tool, based on SEM, is currently best solution for defect inspection.
The ebeam inspection tool is to find or identify defects in the semiconductor process, and relative to review SEM, a large FOV (Field-of-View) and large beam current are commercial means to enhance inspection throughput. In order to obtain large FOV, a SORIL (Swing Objective Retarding Immersion Lens) system is applied commercially. Moreover, resolution is sometimes lowered, compared to review SEM, enough to capture defects.
In electron-beam inspection (EBI), as the image resolution specification is becoming higher and higher, the image vibration needs to be suppressed in lower and lower range, e.g., several nanometers or even sub-nanometer. In this course, position feedback accuracy is critical for the image vibration control, especially, on continuous scanning operation. Currently, high accuracy laser interferometer is commonly applied in the EBI for beam position measurement. Normally, one sinusoidal and one cosinusoidal signals are the output position information from the laser interferometer measurement. These two channel signals need to be interpolated to achieve nanometer or sub-nanometer position accuracy for the image location feedback. Ideally, these two signals from laser interferometer have the same magnitude and 90 degrees phase shift.
However, in reality, there are some differences in magnitude between these two analogue signals. Also, the phase shift between the both is not exact 90 degrees. Therefore, a sub-divisional error (SDE) is generated when these two signals are interpolated. Sub-Divisional Error (SDE) is cyclic and non-accumulative. It results from interpolation errors (not count loss), typically caused by imperfections in the analogue sine and cosine signals fed to the interpolator by the encoder's read head.
Especially on continuous scanning operation with slow moving speed, the single frequency disturbance from SDE is severe and obvious in image vibration analysis. This SDE effect in the laser position measurement needs to be compensated to achieve desired inspection performance.